Field of the Invention
The present invention relates to a power conversion system, and particularly to a power conversion system including a plurality of power converters connected in parallel to a load.
Description of the Background Art
An uninterruptible power supply system including a plurality of uninterruptible power supply devices connected in parallel to a load has been conventionally known. Each uninterruptible power supply device includes a conversion circuit for converting an AC voltage supplied from an AC power supply into a DC voltage, an inversion circuit for converting the DC voltage into an AC voltage and providing the AC voltage to a load, and a DC bus for supplying the DC voltage generated by the conversion circuit to the inversion circuit.
At the normal time when the AC power is supplied from the AC power supply, the AC power is converted into the DC power by the conversion circuit and the DC power is stored in a power storage device, and further, the DC power is converted into the AC power by the inversion circuit and the AC power is supplied to the load. At the time of a power failure when supply of the AC power from the AC power supply is stopped, the DC power of the power storage device is converted into the AC power by the inversion circuit and the AC power is supplied to the load. Therefore, even when a power failure occurs, the operation of the load can be continued during a time period in which the DC power is stored in the power storage device.
Japanese Patent Laying-Open No. 2007-74823 discloses a power converter including a DC positive bus, a DC negative bus, two fuses, and two inverter units. Each inverter unit includes a capacitor and a semiconductor module connected in parallel to each other. A positive side terminal of the semiconductor module is connected to the DC positive bus with the fuse interposed therebetween, and a negative side terminal of the semiconductor module is connected to the DC negative bus. When the semiconductor module of the inverter unit fails and becomes short-circuited, an overcurrent flows and the fuse is blown.
However, the conventional uninterruptible power supply system had a problem that output voltages of the plurality of inversion circuits vary and a cross current flows between output terminals of the plurality of inversion circuits. As a method for dealing with this, it is conceivable to suppress the variation of the output voltages of the plurality of inversion circuits by short-circuiting the plurality of DC buses and making input voltages of the plurality of inversion circuits uniform. According to this method, however, when the conversion circuit or the like of one uninterruptible power supply device fails and becomes short-circuited, it is assumed that an overcurrent flows from the other uninterruptible power supply devices to the short-circuited portion and the other uninterruptible power supply devices also fail.